Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Reexamination Certificate
2002-07-22
2004-02-17
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
C430S620000, C430S631000, C430S964000, C430S965000
Reexamination Certificate
active
06692906
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to aqueous dispersions of silver (carboxylate-azine toner) particles. The carboxylates are typically silver salts of long chain fatty acids and the azine toners are the compounds that function as development accelerators and toning agents. These silver (carboxylate-azine) particles are used to formulate imaging forming compositions that are useful in aqueous photothermographic or thermographic imaging elements. In another aspect, the invention relates to a coprecipitation method for producing the particles.
DESCRIPTION RELATIVE TO THE PRIOR ART
Thermographic and photothermographic materials and imaging elements are well known in the photographic art. These materials are also known as heat developable photographic materials. Thermographic materials can form an image by the imagewise application of heat. Photothermographic materials include a light sensitive material, for example a silver halide. After imagewise exposure photothermographic materials are heated to moderately elevated temperatures to produce a developed image in the absence of separate processing solutions or baths.
An example of a known photothermographic silver halide material comprises (a) a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with (b) an organic solvent mixture, (c) a hydrophobic binder and (d) an oxidation-reduction image-forming composition. The oxidation-reduction imaging forming composition typically comprises (i) a silver carboxylate that is usually a silver salt of a long-chain fatty acid, such as silver behenate or silver stearate, in combination with (ii) an organic reducing agent, such as a phenolic reducing agent. It has been desirable to have hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer in such a photothermographic material because of the higher photosensitivity of the silver halide emulsion and the ease of control in preparation of the emulsion based on conventional aqueous silver halide gelatino emulsion technology.
A problem has been encountered in preparing these photothermographic silver halide materials. This problem involves the mixing of a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with an oxidation-reduction imaging forming composition. The imaging forming composition contains hydrophobic components including a hydrophobic binder, such as poly(vinyl butyral), and a silver salt of a long-chain fatty acid, such as a silver salt of behenic acid. Typically, when the hydrophilic photosensitive silver halide emulsion is mixed with the hydrophobic imaging forming materials and then coated on a suitable support to produce a photothermographic element, the resulting element produces a less than desired degree of photosensitivity, contrast and maximum density upon exposure and heat processing. This problem has been encountered in photothermographic silver halide materials, as described in, for example, U.S. Pat. No. 3,666,477 of Goffe, issued May 30, 1972. Goffe proposed addition of alkylene oxide polymers and a mercaptotetrazole derivative to the photothermographic material to help provide increased photosensitivity. In addition, a variety of organic solvents have been proposed in order to help prepare a photothermographic silver halide composition containing the described image-forming components. The organic solvents that have been proposed include isopropanol, acetone, toluene, methanol, 2-methoxyethanol, chlorinated solvents, acetone-toluene mixtures and certain non-aqueous polar organic solvents. The described individual solvents, such as isopropanol, have not provided the desired improved properties. There has been a continuing need to provide improved relative speed, contrast and image tone with desired maximum image density.
It is known to provide toners in thermographic and photothermographic compositions to increase chemical reactivity of the development chemistry and to improve the tone of the developed image. The compositions described herein are typically used to produce elements that are useful in x-ray imaging. For diagnostic purposes, doctors prefer neutral images on blue tinted support. The images should have very low minimum density and very high maximum density for optimum diagnostic use. The use of toner compounds can help accomplish these objectives.
A variety of toner compositions are known. For example, in EP 0803764 A1 filed Apr. 16, 1997, there is described a thermographic composition having a succinimide toner incorporated in the composition (See Example 1).
The materials and imaging elements described herein can be used as output media and can be exposed using a laser printer, typically from a digitized x-ray image. Laser printers of interest typically expose the elements to infrared laser radiation, for example in the 800 nm range. Since silver halide is not inherently sensitive to infrared radiation, it must be spectrally sensitized to this wavelength range in order to be effectively exposed.
Recent developments have focused on providing imaging compositions, for example photothermographic compositions, that are aqueous based. Such compositions, compared to organic solvent-based compositions, have numerous coating advantages. For example, expensive organic solvent recovery systems are not necessary in the coating process.
In commonly assigned U.S. Pat. No. 5, 350,669 to Witcomb et al, issued Sep. 27, 1994, there are disclosed compositions comprising silver, carboxylate and azine as the primary non-photosensitive, reducible silver source for a photothermographic element. These compounds contain relatively large amounts of the expensive azine component. The minimum amount of azine disclosed by Witcomb et al is about 14% by weight relative to silver carboxylate. (This assumes the minimum mass associated with the azine structure and a maximum for the carboxylate within the ranges specified.)
We have found that the presence of azine toner compounds significantly impacts the ability to spectrally sensitize a photosensitive silver halide emulsion in an aqueous environment. The inability to maintain sufficient spectral sensitization causes it to be difficult to maintain an adequate maximum density in the processed elements. Succinimide toner does not desensitize infrared sensitized silver halide.
SUMMARY OF THE INVENTION
In one aspect of the invention, there is provided an aqueous dispersion of silver-carboxylate particles having incorporated therein an azine toner compound. The azine content of the silver (carboxylate-azine toner) particles is from about 0.01 to 10% by weight relative to silver carboxylate, preferably about 0.05 to 5%. Other species can also be present, for example about 0.01 to 20% by weight relative to the silver carboxylate can be carboxylic acid, preferably 5 to 15% and about 0.01 to 2% by weight relative to the silver carboxylate can be alkali metal carboxylate salt (for example sodium or potassium carboxylate etc.) preferably 0.5 to 1.5%.
As will be seen in the comparative examples below, these silver (carboxylate-azine toner) particles substantially avoid the desensitization of spectrally sensitized silver halide. While not wishing to be bound by any particular theory, we believe that the desensitization by azine toner compounds in prior compositions can be attributed to the desorption of the spectral sensitizing dye from the surface of the silver halide grains. This in turn may be caused by the presence of the “free” azine toner compound. In the present invention, the azine toner is incorporated into the carboxylate particles and is therefore not “free” to desensitize adjacent silver halide grains. These particles provide the desired silver development kinetics, image density and image tone.
As noted, a characteristic of the present invention is that the silver-carboxylate particles have an azine toner compound incorporated into the structure of the particles. Another aspect of the invention is that the azine is present in a small amount. We have found that this small amount provides the de
Blanton Thomas
Boettcher John W.
Dickinson David A.
Ghyzel Peter J.
Klaus Roger L.
Chea Thorl
Eastman Kodak Company
Hawley J. Jeffrey
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